nua/test_refer.c: run test_challenge_refer() only if we use proxy
nua/test_refer.c: run test_challenge_refer() only if we use proxy

Test case is now more deterministic, too.

darcs-hash:20080122174902-65a35-c14a528a6adb43506bd8765ca5ad7270db849945.gz

xhook/sofia-sip,unispeech/sofia-sip,unispeech/sofia-sip,erdincay/sofia-sip,jart/sofia-sip,xhook/sofia-sip,xhook/sofia-sip,BelledonneCommunications/sofia-sip,erdincay/sofia-sip,erdincay/sofia-sip,erdincay/sofia-sip,BelledonneCommunications/sofia-sip,xhook/sofia-sip,jart/sofia-sip,BelledonneCommunications/sofia-sip,BelledonneCommunications/sofia-sip,jart/sofia-sip,unispeech/sofia-sip,xhook/sofia-sip,unispeech/sofia-sip